The invention relates generally to the manufacture of integrated circuits and, more particularly, to a method for improving access to integrated circuit micro-sections.
Measuring the dimensions of features, such as sidewall angles and trenches for devices such as dynamic random access memories (DRAMs), is a critical and difficult task that impacts the design, development and fabrication of integrated circuits (ICs). Critical Dimensions (CDs) are the sizes of the smallest geometrical features, such as the width of interconnect lines, contacts or trenches, that can be formed during integrated circuit device/circuit manufacturing using given technology. Current methods for measuring CDs are either limited by size or result in destruction of the wafer.
Critical Dimension Scanning Electron Microscopy (CD-SEM) is an imaging method with a lateral resolution below 10 nm. A focused beam of electrons is scanned across a sample and an image is constructed based on the detection of secondary electron current. The sample being measured must be coated with a conductive film. Two main types of measurements are employed: 1) top-down SEMs, used for linewidth measurements of features, and 2) feature cross-section measurements. CD-SEMs are able to measure dimensions near the surface of a chip, but they cannot collect secondary electron signals from the bottom of a hole or trench.
Atomic Force Microscopy (AFM) is a method that is capable of surface visualization with near-atomic resolution. AFM provides a measurement of the roughness of solid surfaces based on electrostatic interactions between the surface and the measuring tip. The measuring tip can be set above the surface, on the surface, or can tap the surface, oscillating at high frequency (tapping mode). However, if the openings of the features are small, AFM tips may not be able to reach into them. Currently, sidewall angles for these features are measured from cross-section SEM images. This requires cleaving and destruction of the wafer.
Many features of interest in integrated circuit manufacturing are difficult to measure by using AFMs or top-down CD-SEMs. This is because either the tip of the AFM is too large for the opening of the features or the collection angle for the secondary electrons is steep. There are many such parameters for which measurements are desired, such as sidewall angles and profiles and trench and various recess depths.
Accordingly, there is a need for a method to enable access to small features of integrated circuits, a method that does not result in the destruction of the wafer. The present invention enables access to small integrated circuit features by increasing the size of openings to those features without destroying the wafer. The present invention uses masking and etching to clear sufficiently large areas to allow a measurement tool ingress.
More specifically, the present invention provides a method of enabling measurement access to small integrated circuit features that comprises selecting a feature of an integrated circuit on a wafer and providing access to the selected feature by removing a portion of the integrated circuit adjacent to the feature, thereby preserving the wafer.